Microsoft Knowledge Base Article 815141 contains the following summary:
By default, Internet Explorer Enhanced Security Configuration is enabled on Windows Server 2003. When you start Microsoft Internet Explorer, you receive the following message:
Microsoft Internet Explorer's Enhanced Security Configuration is currently enabled on your server. This enhanced level of security reduces the risk of attack from Web-based content that is not secure, but it may also prevent Web sites from displaying correctly and restrict access to network resources.
Internet Explorer Enhanced Security Configuration
established a configuration for your server and for Microsoft Internet Explorer
that decreases the exposure of your server to potential attacks that can occur
through Web content and application scripts.
As a result, some Web sites may not display or perform as expected.
This article describes the effects of Internet Explorer Enhanced Security Configuration in Windows Server 2003.